1. Field of the Invention
The present invention relates to a liquid crystal display element used for a flat display for a personal computer, a liquid crystal TV, a portable display (including a film substrate), a reflection type cell requiring no backlight, and a transmission type cell such as a projection type cell, etc. In particular, the present invention relates to a liquid crystal display element having a liquid crystal region surrounded by a polymer wall, in which the liquid crystal is twisted by a twisted structure of the polymer wall and a method for producing the same.
2. Description of the Related Art
Basically, a liquid crystal display element with the above-mentioned structure utilizes selective reflectivity of liquid crystal molecules caused by a twisted structure thereof. More specifically, the liquid crystal display element utilizes a transparent state of the liquid crystal molecules under the application of a voltage to a liquid crystal cell and a colored or opaque state of the liquid crystal molecules under the application of no voltage to a liquid crystal cell. Such a liquid crystal display element is well lighted with high contrast and a superior viewing angle characteristic. For these reasons, such a liquid crystal display element can be used for a portable display device and a display device for a plurality of people.
As described below, various display modes have been used in conventional examples of the above-mentioned liquid crystal display element.
Some examples thereof utilize an electro-optic effect. As such elements utilizing an electro-optic effect, Twisted Nematic (TN) type display elements and Super-twisted Nematic (STN) type display elements using nematic liquid crystal have been brought to the market-place. For such elements, polarizing plates are indispensable.
According to other display modes such as a dynamic scattering mode and a phase transition mode, the light scattering of liquid crystal molecules is utilized without using polarizing plates. In recent years, a liquid crystal display element requiring no polarizing plates and no alignment treatment have been proposed in which a transparent state and an opaque state are electrically controlled by using birefringence of liquid crystal. In such an element, the refractive index of an ordinary light is matched with that of a supporting medium, whereby a transparent state is displayed under the condition that liquid crystal molecules are aligned upon being applied with a voltage and a light scattering state (i.e., opaque state) is displayed, using the disorder of the alignment of the liquid crystal molecules under no application of a voltage.
According to still another display mode, a dichroic dye is added to a material for a liquid crystal display element. In such an element, liquid crystal molecules are aligned along polymer walls under no application of a voltage, whereby a director of the dichroic dye becomes random to display a black state. Under the application of a voltage, the liquid crystal molecules are aligned in the direction perpendicular to that of electrodes, whereby a weakly-colored state is displayed. Such a display mode is proposed in Japanese National Publication No. 61-502128. According to this Publication, a mixture containing liquid crystal and polymerizable or thermosetting material is injected between a pair of substrates, and the resin is cured by light-irradiation or by heating to allow the liquid crystal to deposit, thereby forming liquid crystal droplets in the resin.
According to still another display mode, a liquid crystal material containing nematic liquid crystal with a chiral agent added thereto, or cholesteric liquid crystal is used to cause selective reflection of liquid crystal molecules. Although a liquid crystal display element using such a material is excellent in display quality, it requires a high drive voltage (i.e., tens of voltages), making it difficult to put the element into practical use. Furthermore, Japanese Laid-Open Patent Publication No. 3-209425 discloses a liquid crystal display element in which the liquid crystal material containing nematic liquid crystal with a chiral agent added thereto or cholesteric liquid crystal is confined in a polymer. The liquid crystal display element disclosed in this publication is produced by injecting a mixture containing a liquid crystal material, a polymerizable material, a chiral agent, and a photopolymerization initiator between a pair of facing substrates, irradiating UV-rays to the mixture so as to allow the mixture to be photo-polymerized, whereby the mixture is phase-separated into a polymer material and chiral nematic liquid crystal. In such an element, a colored state (i.e., a state in which a region of chiral nematic liquid crystal selectively reflects light) and an opaque state are electrically controlled. In this element, helical rods of chiral nematic liquid crystal move in group; therefore, a high drive voltage is required, making it difficult to put the element into practical use.
Japanese Laid-Open Patent Publication No. 5-27242 proposes a non-light scattering display mode in which the viewing angle characteristic of a liquid crystal cell is improved by using polarizing plates. According to this display mode, a mixture containing liquid crystal and a polymerizable material is injected between a pair of substrates, the mixture is irradiated with light to be phase-separated, whereby a display medium made of a composite material containing liquid crystal and a polymer material is produced. In this display mode, the alignment of liquid crystal domains becomes random due to the polymer material thus generated and liquid crystal molecules in each domain rise in different directions under the application of a voltage. Therefore, apparent refractive indices from the respective directions become equal, and the viewing angle characteristic in an intermediate state (i.e., a state between a transparent state and an opaque state) can be improved.
Furthermore, recently, the inventors of the present application have proposed a liquid crystal display element in which the irradiation of light during photopolymerization is controlled by using a photomask, whereby liquid crystal domains are aligned in an omnidirection (i.e., radial direction) in each pixel region (Japanese Patent Application No. 4-286487). In this liquid crystal display element, the viewing angle characteristic is improved by controlling the alignment of liquid crystal molecules with a voltage.
In spite of the above-mentioned advantages of the respective display modes, the following problems remain unsolved.
(i) In the liquid crystal display element utilizing the selective reflection of a cholesteric phase or a chiral nematic phase, the liquid crystal regions keep energy for spiraling themselves. Therefore, an intensive electric field is required for removing such a helical state from the liquid crystal regions to allow the liquid crystal molecules in the regions to be aligned in the direction perpendicular to the electric field. Thus, such elements lack practical values. PA1 (ii) In the liquid crystal display element using liquid crystal with a dichroic dye added thereto, the dichroic ratio of the dye cannot be utilized to the full, resulting in a dark and low contrast. PA1 (iii) In the ordinary display modes (e.g., TN, STN, FLC, and ECB modes), sufficient cell thickness cannot be maintained in the display regions. Therefore, the cell thickness is likely to be changed by external force, and an ununiform display locally occurs due to a pen input. PA1 wherein the liquid crystal region is in a selective reflection state and a chiral pitch of liquid crystal contained in the liquid crystal regions is longer than a chiral pitch of the walls made from a polymer material. PA1 injecting a mixture containing a liquid crystal material, a liquid crystalline compound having polymerizable functional groups in its molecules, a polymerizable compound, and a photopolymerization initiator between a pair of substrates, either one of the substrates being transparent and each of the substrates having electrodes, molecules having a chiral center being added to either one of the liquid crystal material or the liquid crystalline compound; and PA1 irradiating light having regular strong and weak portions to the mixture to phase-separate the mixture into a liquid crystal material and a polymer material by photopolymerization, thereby allowing at least part of liquid crystal regions to be surrounded by walls made from the polymer material.